Krallen is a unique Monster. Krallen can be sent out with your other Champion, making Krallen somewhat a Semi-Champion, because although Krallen has some Champion traits, Krallen does not have some of the Champions' characteristics like how Krallen cannot be frozen, or how Krallen can be flinged with other Champions as previously mentioned.
Krallen, once paired alongside your other Champion, will result in a deadly combo. If your Krallen is paired Drull, Gorgo , or Korath, and your Krallen has loot buff, the Champion will be able to loot more resources with its damage. This also goes for other Monsters, which are also able to loot more once paired with Krallen.
Although when attacking with Krallen, one should send in a high health monster like Gorgo, Korath or some D.A.V.Es to stall tower damage. Krallen has somewhat lacking damage, so you will also need to send some high damage monster like D.A.V.Es and such. Be aware that Krallens favorite target are Resource Buildings, so Krallen is vunerable to NeCoRG, and Silo Death-Traps.
Krallen can also be used with Fomor. The two compliment each other very well, Fomor buffs Krallen, while Krallen also buffs Fomor. It's a mutual effect.
Although Krallen provides an extra defense for your yard, Krallen will not do well in defending, as some Monsters can kill it quite easily. Its common threats are D.A.V.Es, Balthazars, Banditos, Wormzers with splash damage, and KW. Champions can also kill Krallen very easily.
Krallen, if its partner is Fomor, may be effective at defending as Fomor buffs Krallen, making it harder to kill. However, if the Fomor is down, it will not be long before Krallen is down aswell.